1. Field of the Invention
This invention pertains in general to the field of surface plasmon resonance (SPR) spectroscopy. In particular, the invention relates to a novel SPR approach involving the coupling of plasmon resonances in a thin metal film and the waveguide modes in a dielectric overcoating in the ultraviolet and infrared electromagnetic spectral ranges.
2. Description of the Related Art
Surface plasmon resonance is a phenomenon used in many analytical applications in metallurgy, microscopy, and chemical and biochemical sensing. With optical techniques such as ellipsometry, multiple internal reflection spectroscopy, and differential reflectivity, SPR is one of the most sensitive techniques to surface and interface effects. This inherent property makes SPR well suited for nondestructive studies of surfaces, interfaces, and very thin layers. SPR is also used in other than surface investigations and it has recently been demonstrated as a new optical technique for use in immunoassays.
The SPR phenomenon has been known for decades and the theory is fairly well developed. Simply stated, a surface plasmon is an oscillation of free electrons that propagates along the surface of a conductor. The phenomenon of surface plasmon resonance occurs under total internal reflection conditions at the boundary between substances of different refractive indices, such as glass and water solutions. When an incident light beam is reflected internally within the first medium, its electromagnetic field produces an evanescent wave that crosses a short distance (in the order of nanometers) beyond the interface with the second medium. If a thin metal film is inserted at the interface between the two media, surface plasmon resonance occurs when the free electron clouds in the metal layer (the plasmons) absorb energy from the evanescent wave and cause a measurable drop in the intensity of the reflected light at a particular angle of incidence that depends on the refractive index of the second medium.
Typically, the conductor used for SPR spectrometry is a thin film of metal such as silver or gold; however, surface plasmons have also been excited on semiconductors. The conventional method of exciting surface plasmons is to couple the transverse-magnetic (TM) polarized energy contained in an evanescent field to the plasmon mode on a metal film. The amount of coupling, and thus the intensity of the plasmon, is determined by the incident angle of the light beam and is directly affected by the refractive indices of the materials on both sides of the metal film. By including the sample material to be measured as a layer on one side of the metallic film, changes in the refractive index of the sample material can be monitored by measuring changes in the surface plasmon coupling efficiency in the evanescent field. When changes occur in the refractive index of the sample material, the propagation of the evanescent wave and the angle of incidence producing resonance are affected. Therefore, by monitoring the angle of incidence at a given wavelength and identifying changes in the angle that causes resonance, corresponding changes in the refractive index and related properties of the material can be readily detected.
As those skilled in the field readily understand, total reflection can only occur above a particular critical incidence angle if the refractive index of the incident medium (a prism or grating) is greater than that of the emerging medium. In practice, total reflection is observed only for incidence angles within a range narrower than from the critical angle to 90 degrees because of the physical limitations inherent with the testing apparatus. Similarly, for systems operating with variable wavelengths and a given incidence angle, total reflection is also observed only for a corresponding range of wavelengths. This range of incidence angles (or wavelengths) is referred to as the "observable range" for the purpose of this disclosure. Moreover, a metal film with a very small refractive index (as small as possible) and a very large extinction coefficient (as large as possible) is required to support plasmon resonance. Accordingly, gold and silver are appropriate materials for the thin metal films used in visible-light SPR; in addition, they are very desirable because of their mechanical and chemical resistance.
Thus, once materials are selected for the prism, metal film and emerging medium that satisfy the described conditions for total reflection and plasmon resonance, the reflection of a monochromatic incident beam becomes a function of its angle of incidence and of the metal's refractive index, extinction coefficient, and thickness. The thickness of the film is therefore selected such that it produces observable plasmon resonance when the monochromatic light is incident at an angle within the observable range.
The classical embodiments of SPR devices are the Kretschmann and Otto prism or grating arrangements, which consist of a prism with a high refractive index n (in the 1.4-1.7 range) coated on one face with a thin film of metal. The Otto device also includes a very thin air gap between the face of the prism and the metal film. In fact, the gap between the prism (or grating) and the metal layer, which is in the order of nanometers, could be of a material other than air, even metal, so long as compatible with the production of observable plasmon resonance in the metal film when the monochromatic light is incident at an angle within the observable range.
Similar prior-art SPR devices are based on the phenomenon of long-range surface plasmon resonance, which is also generated with p-polarized light using a dielectric medium sandwiched between the incident medium and a thinner metal layer (than in conventional SPR applications). The metal film must be sufficiently thin and the dielectric and emergent media must be beyond the critical angle (i.e., having refractive indices smaller than the refractive index of the entrant medium) so that they support evanescent waves to permit the simultaneous coupling of surface plasmons at the top and bottom interfaces of the thin metal layer (i.e., to permit excitation of surface waves on both sides of the thin metal film). This condition is necessary in order for the phenomenon of long-range surface plasmon resonance to occur. For a given set of parameters, the distinguishing structural characteristic between conventional surface plasmon resonance and long-range surface plasmon resonance is the thickness of the metal film and of the inner dielectric film (the latter not being necessary for conventional SPR). In the conventional technique, the metal film must be sufficiently thick and must be placed either directly on the entrant medium (i.e., prism or grating), or on a dielectric film which is too thin, to allow excitation of the surface bound waves on both metal surfaces to produce observable plasmon resonance when a monochromatic light is incident at an angle within the observable range. In long-range surface plasmon resonance, in contrast, the metal film must be placed between two dielectric media that are beyond the critical angle so that they support evanescent waves, and must be thin enough to permit excitation of surface waves on both sides of the metal film. The specific thickness depends on the optical parameters of the various components of the sensor in question, but film thicknesses in the order of 45-55 nm for gold and silver are recognized as critical for conventional SPR, while no more than about half as much (15-28 nm) can be used for LRSPR. It is noted that the thickness required to support either form of surface plasmon resonance for a specific system can be calculated by one skilled in the art on the basis of the system's optical parameters.
As well understood by those skilled in the art, the main criterion for a material to support SP waves is that it have a negative real dielectric component, which results from the refractive and extinction properties mentioned above for the metal layer. The surface of the metal film forms the transduction mechanism for the SPR device and is brought into contact with the sample material to be sensed at the interface between the metal film and the emerging medium contained in a cell assembly. Monochromatic light is emitted by a laser or equivalent light source into the prism or grating and reflected off the metal film to an optical photodetector to create the sensor output. The light launched into the prism and coupled into the SP mode on the film is p-polarized with respect to the metal surface where the reflection takes place. In all these prior-art devices and techniques, only p-polarized light is coupled into the plasmon mode because this particular polarization has the electric field vector oscillating normal to the plane that contains the metal film. This is sometimes referred to as transverse-magnetic (TM) polarization.
As mentioned, the surface plasmon is affected by changes in the dielectric value of the material in contact with the metal film. As this value changes, the conditions necessary to couple light into the plasmon mode also change. Thus, SPR is used as a highly sensitive technique for investigating changes that occur at the surface of the metal film. In particular, over the last several years there has been a keen interest in the application of surface plasmon resonance spectroscopy to study the optical properties of molecules immobilized at the interface between solid and liquid phases. The ability of the SPR phenomenon to provide information about the physical properties of dielectric thin films deposited on a metal layer, including lipid and protein molecules forming proteolipid membranes, is based upon two principal characteristics of the SPR effect. The first is the fact that the evanescent electromagnetic field generated by the free electron oscillations decays exponentially with penetration distance into the emergent dielectric medium; i.e., the depth of penetration into the material in contact with a metal layer extends only to a fraction of the light wavelength used to generate the plasmons. This makes the phenomenon sensitive to the optical properties of the metal/dielectric interface without any interference from the properties of the bulk volume of the dielectric material or any medium that is in contact with it. The second characteristic is the fact that the angular (or wavelength) position and shape of the resonance curve is very sensitive to the optical properties of both the metal film and the emergent dielectric medium adjacent to the metal surface. As a consequence of these characteristics, SPR is ideally suited for studying both structural and mass changes of thin dielectric films, including lipid membranes, lipid-membrane/protein interactions, and interactions between integral membrane proteins and peripheral, water-soluble proteins. See Salamon, Z., H. A. Macleod and G. Tollin, "Surface Plasmon Resonance Spectroscopy as a Tool for Investigating the Biochemical and Biophysical Properties of Membrane Protein Systems. I: Theoretical Principles," Biochim. et Biophys. Acta, 1331: 117-129 (1997); and Salamon, Z., H. A. Macleod and G. Tollin, "Surface Plasmon Resonance Spectroscopy as a Tool for Investigating the Biochemical and Biophysical Properties of Membrane Protein Systems. II: Applications to Biological Systems," Biochim. et Biophys. Acta, 1331: 131-152 (1997).
In U.S. Pat. No. 5,991,488, we disclosed new thin-film interface designs that couple surface plasmon and waveguide excitation modes. The new technique, defined as coupled plasmon-waveguide resonance (CPWR), is based on the totally new concept of coupling plasmon resonances in a thin metal film with the waveguide modes in a dielectric overcoating. Accordingly, a metallic layer, typically either gold or silver, is used with a prism so as to provide a surface plasmon wave by conventional SPR (or waves by long-range SPR) and is further covered with a solid dielectric layer characterized by predetermined optical parameters. The dielectric member inserted between the metal film and the emergent medium is selected such that coupled plasmon-waveguide resonance effects are produced within an observable range. The emergent dielectric medium is then placed in contact with this solid dielectric layer. We found that the additional layer of dielectric material functions as an optical amplifier that produces an increased sensitivity and enhanced spectroscopic capabilities in SPR. In particular, the added dielectric layer makes it possible to produce resonance with either s- or p-polarized light. In addition, the added dielectric protects the metal layer and can be used as a matrix for adsorbing and immobilizing the sensing materials in sensor applications. This disclosure provides a further improvement in coupled plasmon-waveguide resonance that constitute material advances in the art of SPR.